1. Field of the Invention
Generally, the present invention relates to the formation of integrated circuits, and, more particularly, to the formation of metallization layers including highly conductive metals, such as copper, embedded into a dielectric material having a low permittivity to enhance device performance.
2. Description of the Related Art
In an integrated circuit, a very large number of circuit elements, such as transistors, capacitors, resistors and the like, are formed in or on an appropriate substrate, usually in a substantially planar configuration. Due to the large number of circuit elements and the required complex layout of the integrated circuits, the electrical connection of the individual circuit elements are generally not established within the same level on which the circuit elements are manufactured. Typically, such electrical connections are formed in one or more additional “wiring” layers, also referred to as metallization layers. These metallization layers generally include metal-containing lines, providing the inner-level electrical connection, and also include a plurality of inter-level connections, also referred to as vias, filled with an appropriate metal. The vias provide electrical connection between two neighboring stacked metallization layers, wherein the metal-containing lines and vias may also be commonly referred to as interconnects.
Due to the continuous shrinkage of the feature sizes of circuit elements in modem integrated circuits, the number of circuit elements for a given chip area, that is the packing density, also increases, thereby requiring an even larger increase in the number of electrical interconnections to provide the desired circuit functionality. Therefore, the number of stacked metallization layers may increase and the dimensions of the individual lines and vias may be reduced as the number of circuit elements per chip area becomes larger. The fabrication of a plurality of metallization layers entails extremely challenging issues to be solved, such as mechanical, thermal and electrical reliability of a plurality of stacked metallization layers. As the complexity of integrated circuits advances and brings about the necessity for conductive lines that can withstand moderately high current densities, semiconductor manufacturers are increasingly replacing the well-known metallization metal aluminum with a metal that allows higher current densities and hence allows a reduction in the dimensions of the interconnections and thus the number of stacked metallization layers. For example, copper is a metal generally considered to be a viable candidate for replacing aluminum, due to its superior characteristics in view of higher resistance against electromigration and significantly lower electrical resistivity when compared with aluminum. Despite these advantages, copper also exhibits a number of disadvantages regarding the processing and handling of copper in a semiconductor facility. For instance, copper may not be efficiently applied onto a substrate in larger amounts by well-established deposition methods, such as chemical vapor deposition (CVD), and also may not be effectively patterned by the usually employed anisotropic etch procedures. Consequently, in manufacturing metallization layers including copper, the so-called damascene technique (single and dual) is therefore preferably used, wherein a dielectric layer is first applied and then patterned to receive trenches and/or vias which are subsequently filled with copper or copper alloys.
The process of filling copper or copper alloys into highly scaled openings, such as trenches or vias having aspect ratios (depth/diameter) of approximately 5 or even more for sophisticated integrated circuits, is an extremely challenging task for process engineers. As previously noted, copper and its respective alloys may not efficiently be deposited by chemical or physical vapor deposition and hence copper-based metals are typically deposited by electrochemical techniques, such as electroless plating or electroplating. Although electro-plating techniques for depositing copper are well established in the field of manufacturing integrated circuit boards, completely new deposition techniques have been developed for the formation of copper-based metallization layers in accordance with the damascene technique regarding fill behavior during the copper deposition, in which trenches and vias are filled substantially from bottom to top with a minimum number of defects, such as voids within the trenches and vias. After the deposition of the copper or copper-based metal, the excess material deposited on areas outside of the trenches and vias has to be removed, which is currently accomplished by chemical mechanical polishing (CMP), possibly in combination with electrochemical etch techniques.
In highly advanced semiconductor devices, the dielectric material in which the copper-based metal is embedded typically comprises a so-called low-k material, that is a material having a relative permittivity significantly lower than “conventional” dielectric materials, such as silicon dioxide, silicon nitride and the like, so that, in general, the relative permittivity of the low-k material is 3.0 or even less. However, the reduced permittivity usually comes along with a significantly reduced mechanical strength and stability. Therefore, in typical damascene techniques for forming low-k metallization layers of advanced semiconductor devices, a capping layer is provided that ensures the mechanical integrity of the low-k dielectric material, thereby acting as a polish stop layer during the removal of the excess metal.
With reference to FIGS. 1a-1c, a typical conventional process flow will now be described in more detail to more clearly demonstrate the problems involved in forming highly scaled copper lines in a low-k dielectric material.
FIG. 1a schematically shows a cross-sectional view of a semiconductor device 100 comprising a substrate 101, which may be provided in the form of a bulk silicon substrate, a silicon-on-insulator (SOI) substrate and the like, wherein the substrate 101 may also represent a device layer having formed therein individual circuit elements, such as transistors, capacitors, lines, contact portions and the like. For convenience, any such circuit elements are not shown in Figure la. The device 100 further comprises a dielectric layer 102 formed above the substrate 101, wherein the layer 102 may represent a dielectric material enclosing the individual circuit elements, or the layer 102 may represent a portion of a lower-lying metallization layer, in which any metal-filled vias (not shown) may be embedded. Depending on the specific design of the device 100, or the function of the layer 102, it may be comprised of a conventional dielectric material such as silicon dioxide, silicon nitride, or it may comprise a low-k dielectric material such as, for instance, hydrogen-enriched silicon oxycarbide (SiCOH).
A further dielectric layer 103 is formed above the layer 102 and is comprised of a low-k dielectric material which helps to reduce the parasitic capacitance between adjacent metal lines to be formed in the low-k dielectric layer 103. As previously pointed out, the mechanical strength of the layer 103 is typically reduced compared to materials such as silicon dioxide and silicon nitride and therefore usually a capping layer 104 is provided on top of the layer 103 to provide the integrity of the low-k dielectric layer 103 in subsequent manufacturing processes. Furthermore, a dielectric layer 105 designed to act as an anti-reflective coating (ARC) layer is formed on the capping layer 104 followed by a further capping layer 106 that is substantially devoid of any nitrogen. A resist mask 107 is formed above the layer 106 and has formed therein an opening that substantially corresponds to a trench 108 formed in the layers 106, 105, 104 during an etch process 109.
Typically, the layer 105 may comprise nitrogen, for instance in the form of silicon oxynitride, which may adversely affect the performance of the resist mask 107, as modem photoresists are extremely affected by nitrogen and nitrogen compounds in that the sensitivity to the exposure wavelength is significantly reduced owing to a chemical reaction in the photoresist caused by the presence of nitrogen and nitrogen radicals. This effect, referred to as resist poisoning, has long been under-estimated but is gaining in importance as the dimensions of the trench 108 are continuously scaled down. The effect of resist poisoning may result in incompletely removed resist portions in the corresponding opening of the resist mask 107, thereby finally causing irregularities during the etch process for the trench 108. Consequently, the capping layer 106 is provided, for instance in the form of silicon dioxide, to act as a buffer layer between the resist mask 107 and the ARC layer 105.
A typical process flow for forming the semiconductor device 100 as shown in FIG. 1a may comprise the following processes. After the completion of any circuit elements within the substrate 101, the dielectric layer 102 may be deposited by well-established deposition recipes based on plasma enhanced chemical vapor deposition (PECVD). For example, the layer 102 may be comprised of silicon dioxide or fluorine-doped silicon dioxide and hence deposition recipes on the basis of TEOS may be employed to form the layer 102. It should be noted that any additional etch stop layers and the like may be included in the layer 102. Similarly, depending on process requirements, dielectric etch stop layers or barrier layers may be formed on an upper portion of the layer 102.
Thereafter, the low-k dielectric material is deposited to form the layer 103. For instance, SiCOH is a well-established low-k material that may be deposited by PECVD on the basis of precursors, such as 3 MS (trimethylsilane), 4 MS and the like. However, other materials may be used which may be applied on the basis of spin-on techniques and the like.
Thereafter, the capping layer 104 is formed, for instance in the form of silicon dioxide, by means of well-established PECVD techniques based on precursor materials such as silane and the like. Next, the ARC layer 105 is deposited on the basis of PECVD techniques, wherein the thickness and the material composition is selected to provide, in combination with the layer 106, the desired optical behavior as an anti-reflective coating layer. For instance, the index of refraction of the ARC layer 105 may be adjusted by adjusting the amount of nitrogen during the deposition of silicon dioxide. Thereafter, the capping layer 106 is deposited, for instance in the form of silicon dioxide, by PECVD.
Next, the resist mask 107 is formed by applying a photoresist layer, exposing the layer to a specified exposure wavelength and developing the exposed layer to provide the patterned mask layer 107. Thereafter, the etch process 109 is performed wherein, in an initial phase, the exposed portion of the layer 106, the layer 105 and the layer 104 are removed and, in a subsequent process, the low-k dielectric material is removed to form the trench 108 having moderately steep sidewalls due to the highly anisotropic nature of the etch process.
It should be noted that the initial phase for etching through the layers 106, 105, 104 may require a different etch chemistry compared to the main etch for removing the low-k dielectric material due to differences in material composition, density and the like. It should also be noted that during the etch process 109, a portion of the resist mask 107 is also consumed, wherein, typically, a removal rate for the resist mask 107 is reduced compared to the materials of the layers 103, 104, 105, 106. Otherwise the resist mask 107 may be provided with a sufficient thickness to “overcompensate” the material loss and to serve as an etch mask during the entire etch process if the removal rates are comparable. Thereafter, the remaining photoresist is removed and the manufacturing process is continued with the deposition of conductive barrier layers, a seed layer and the electrochemical deposition of the bulk metal by means of, for example, electroplating.
FIG. 1b schematically shows the semiconductor device 100 after the above-described process sequence. Hence, the device 100 comprises a conductive barrier layer 110, which may include one or more sub-layers on the basis of materials reducing the out-diffusion of copper into the surrounding dielectric material and enhancing the adhesion of copper within the low-k dielectric layer 103. Moreover, a copper layer or a copper alloy layer 111 is formed above the layer 110 and within the trench 108. During the deposition of the layer 111, the composition and the kinetics within an electrolyte bath are controlled to yield a highly non-conformal deposition behavior so that, in principle, the copper or copper-based alloy is deposited from bottom to top within the trench 108. For a trench having a high aspect ratio, i.e., the ratio of trench depth to trench width, even slight overhangs at the trench edge 108a may lead to the creation of defects, such as voids 112 within the trench 108, which may finally lead to reliability concerns of the metal-filled trench 108.
FIG. 1c schematically shows the device 100 with the excess material of the layer 111 and the layers 110 removed and also with the layers 106 and 105 removed, which may be accomplished, as previously explained, at least partially by chemical mechanical polishing, during which the layer 104 also acts as a stop layer. Consequently, the layer 104 is also reduced in thickness and is indicated as 104a. Due to the voids 112, the reliability of metallization layers is significantly affected since the corresponding line, i.e., the metal-filled trench 108, may have a reduced conductivity and may also exhibit increased current or temperature-induced material transport, i.e., electromigration, at elevated current densities as are typically encountered in highly scaled devices.
In view of the situation described above, there exists a need for an improved technique which overcomes, or at least reduces the effects of, one or more of the problems set forth above.